Internal combustion engines include exhaust control mechanisms for controlling the flow of exhaust from each combustion chamber of the engine. In most instances, these mechanisms comprise valves.
In those instances where the engine operates on a two-stroke or two-cycle principal, the valve is often of the sliding or rotating type. Generally, this type of valve does not serve to ever completely close the opening or port in the combustion chamber wall or the exhaust passage. Instead, the valve moves between a first position in which the valve does not obstruct or obstructs very little of the exhaust port, and a second position in which the valve partly obstructs the port. In the first position, the timing of the opening of the exhaust port as the piston moves down is advanced, and the timing of the closing of the exhaust port as the piston moves up is delayed, providing for a longer exhaust duration as compared to that when the exhaust timing valve is in the second position. By moving the exhaust timing valve between its first and second positions or positions therebetween, the timing and duration of the exhaust flow, and thus the performance or running conditions of the engine, can be adjusted.
In many instances, an engine employing this type of exhaust valve is used to power a water propulsion device of a planing type boat. The exhaust timing valve of this type of engine is conventionally operated in the following manner.
When the boat is being operated at speed below a predetermined high speed (this speed normally being higher than the speed at which the boat planes), the exhaust valve is moved to its substantially closed second position. In this position, the compression ratio and combustion time are increased, providing high engine power and torque. In addition, blow-by of undesirable emissions such as unburned fuel is reduced.
When the boat speed increases above the predetermined high speed, the engine speed must similarly be increased. At this time, the exhaust valve is moved to its open first position. In this position the exhaust gases flow more freely from the combustion chamber, lowering exhaust resistance and permitting higher engine speeds.
The problem arises that the engine speed in this type of boat may quickly move back and forth between speeds above and below the predetermined speed at which the position of the exhaust timing valve is changed. For example, a rider may accelerate the boat towards a wave. If the boat becomes airborne and the resistance on the water propulsion device is reduced, the engine speed is likely to spike quickly above the predetermined high speed. The exhaust valve is then moved from the second to the first position. Once the boat re-enters the water, however, a high load is placed on the engine and its speed quickly drops back below the predetermined speed. The exhaust valve is then moved back to the second position.
This arrangement is detrimental due to the frequent movement of the exhaust timing valve. The timing of this movement may affect boat performance. For example, when the exhaust timing valve is in the first position when the boat enters the water in the above-described arrangement and the load is placed on the engine, the engine may stall or the boat may at least be sluggish until the exhaust timing valve is repositioned.
An engine control for an engine powering a planing-type boat which overcomes the above-stated problems is desired.